1. Field of the Invention
The present invention relates to a non-electric delay detonator, and to an assembly of a delay detonator and a low-energy detonating cord adapted to actuate the detonator.
2. Description of the Prior Art
The hazards associated with the use of electrical initiation systems for detonating explosive charges in mining operations, i.e., the hazards of premature initiation by stray or extraneous electricity from such sources as lightning, static, galvanic action, stray currents, radio transmitters, and transmission lines, are well-recognized. For this reason, nonelectric initiation through the use of a suitable detonating fuse or cord has been looked upon as a widely respected alternative. A typical high-energy detonating cord has a uniform detonation velocity of about 6000 meters per second and comprises a core of 6 to 10 grams per meter of pentaerythritol tetranitrate (PETN) covered with various combinations of materials, such as textiles, waterproofing materials, plastics, etc. However, the magnitude of the noise produced when a cord having such PETN core loadings is detonated on the surface of the earth, as in trunklines, often is unacceptable in blasting operations in developed areas. Also, the brisance (shattering power) of such a cord may be sufficiently high that the detonation impulse can be transmitted laterally to an adjacent section of the cord or to a mass of explosive which, for example, the cord contacts along its length. In the latter situation, the cord cannot be used to initiate an explosive charge in a borehole at the bottom (the "bottomhole priming" technique), as is sometimes desired.
Low-energy detonating cord (LEDC) was developed to overcome the problems of noise and high brisance associated with the above-described 6-10 grams per meter cord. LEDC has an explosive core loading of only about 0.02 to 2 grams per meter of cord length, and often only about 0.4 grams per meter. This cord is characterized by low brisance and the production of little noise, and therefore can be used as a trunkline in cases where noise has to be kept to a minimum, and as a downline for the bottom hole priming of an explosive charge.
Until recently, most LEDC described in the art had a continuous core of a granular cap-sensitive high explosive such as PETN heavily confined in a metal sheath or one or more woven textile sheaths. An improved LEDC which is light-weight, flexible, strong, and non-conductive, detonates at high velocity, and is readily adapted to high-speed continuous manufacturing techniques is described in Belgian Pat. No. 863,290, granted July 25, 1978, the disclosure of which is incorporated herein by reference. This improved cord has a continuous solid core of a deformable bonded detonating explosive composition comprising a crystalline high explosive compound admixed with a binding agent, and a protective plastic sheath enclosing the core, no metal or woven textile layers being present around the core or sheath. Preferably, one or more continuous strands of reinforcing yarn, e.g., running substantially parallel to the core's longitudinal axis, are present outside the core. The loading of crystalline high explosive in the bonded explosive core is about from 0.1 to 2 grams per meter of length.
Because the low explosive loading of the LEDC core is insufficient to cause the detonation of explosive charges conventionally used in blasting, auxiliary means are used to relay the detonation stimulus from the cord to the charges to be initiated. Delay detonators, actuated by LEDC downlines, are used for this purpose, providing also a delay interval between the initiation of multiple charges. One such detonator, described in U.S. Pat. No. 3,021,786, has an open-ended metal capsule therein enclosing an air gap and having a central aperture. The air gap is between the end of a length of LEDC and an exothermic-burning delay composition, the initiating impulse from the detonation of the explosive core of the LEDC jumping the air gap, passing through the aperture, and igniting the exothermic-burning composition. Peripheral crimps in the side wall of the detonator shell hold the cord in place, the latter forming a plug closure at the open end of the shell, so that the cord and detonator are formed into a composite unit or assembly at the time that the detonator is manufactured.
In the cord/detonator assembly described in U.S. Pat. No. 3,122,097, the section of LEDC extending into the detonator shell has a lower looped portion positioned adjacent to the metal capsule that forms the air gap. The looped portion of cord is positioned in the shell by means of a resilient plug which is crimped in place so as to close off the open end of the shell. Here, too, the cord and detonator are formed into a composite unit or assembly at the time that the detonator is manufactured. In this assembly, the looped section of cord is said to provide improved initiation characteristics possibly due to increased confinement of the explosive core of a metal-sheathed LEDC.
In the cord/detonator assembly of U.S. Pat. No. 3,709,149, the cord is disposed outside a closed shell that contains an impact-sensitive ignition composition, held, for example, in an empty primed rim-fired or center-fired rifle cartridge casing used as an end closure for a detonator. The cord is external in its entirety to the closed interior of the detonator shell, and its end or side is in direct and abutting contact with the exterior surface of the primer end, thereby permitting utilization of either the side or end output of the cord for ignition.
Another LEDC-actuated delay detonator is described in Canadian Pat. No. 627,435. This detonator has an impact- or friction-sensitive ignition charge abutting one side of a diaphragm that can be deformed by a weak blow. The end of a length of LEDC is to be crimped into the open end of the detonator shell with its end abutting the other side of the diaphragm. The ignition of the ignition charge by the detonation of the cord abutting the diaphragm occurs by impact or friction in a manner analogous to the blow given by the firing pin of a shot gun, in contrast to ignition by heat or flame. The diaphragm fits tightly in a pocket, and the latter similarly in the detonator shell. Although this detonator is said to be capable of assembly with the cord in the field by crimping, field assembly of such a detonator presents certain problems. The necessary abutment of the cord end against the diaphragm may not always be achieved in practice in the field owing to the vagaries associated with different handlers, the effects of weather, etc. Also, foreign material could enter the shell prior to insertion of the cord, preventing the necessary cord-to-diaphragm abutting relationship. Dislodgement of the diaphragm-pocket unit also is a possibility.
LEDC-actuated delay detonators are also described in U.S. Pat. No. 3,306,201, wherein the ignition composition also is actuated by percussion of the detonation stimulus from the cord against an imperforate partition.